Megasonic cleaning systems for cleaning semiconductor wafers utilize megasonic energy from transducers which cause a vibratory beam to be propagated into certain chemical cleaning solutions. Megasonic transducers produce a focused beam and create high shear forces at the wafer surface to assist in removing particles from the surface of semiconductor wafers during all processing steps. The beam of energy also removes organic surface films, ionic impurities, and many other contaminants.
Some prior art cleaning systems for semiconductor wafers employ a megasonic transducer at the bottom of a cleaning vessel containing a cleaning solution. A cassette-type holder carries a plurality of wafer substrates in spaced, parallel relation within the vessel. Since the megasonic transducer beam must pass through the cassette holder to engage the wafers, the energy transferred is dissipated by the holder. Thus, the cassette holder reduces the amount of energy transferred to the surfaces of the wafers. Further, the transducer beam does not cover the entire surface area of the wafers and overlaps the wafers in some regions. Thus, some foreign particles are not dislodged by the prior art systems.
In other arrangements similar to the above-mentioned cleaning systems, it is necessary to move the cassette-type holder within the vessel in order to increase the exposure of the surfaces of the wafers to the megasonic energy. An apparatus is needed for moving the holder within the vessel, together with controls for controlling the rate and duration of the movement. Both the moving apparatus and the controls add considerable expense to the cleaning system. Further, since the container must be sufficiently large to accommodate this movement of the holder, vessel expense is significant, and more importantly, it is necessary to provide additional cleaning solution within the vessel.
In yet another prior art megasonic cleaning system, a module is specifically designed to process a single wafer substrate. The module comprises a substrate chamber closely sized to the dimensions of the wafer substrate for maintaining the substrate immersed in a liquid cleaning solution. A high frequency sonic wave generating transducer is maintained outside the substrate chamber for vibrating the cleaning solution at ultrasonic frequencies. The substrate chamber is provided with membranes formed of a material, such as FEP Teflon.RTM., which is inert to the cleaning solutions being used. The single wafer is supported within the chamber on its periphery by support members to minimize surface contact and have the surfaces of the wafer substrate exposed to the cleaning solution and the sonic wave.
A first disadvantage with this particular megasonic cleaning system is that the module is specifically designed to process only one wafer substrate at a time. To process a plurality of wafer substrates simultaneously, a plurality of individual modules must be incorporated into a single processing system. However, this would result in a complex system involving high costs and multiple, time consuming processes. A second disadvantage involves the transducer having to transmit a beam of energy through the support members. This causes the energy transmitted to be dissipated and, therefore, reduces the cleaning efficiency of the beam of energy.
The present invention overcomes many of the disadvantages inherent in the above-described prior art cleaning systems by providing a megasonic cleaning system which can simultaneously process a plurality of wafer substrates. The megasonic cleaning system of the present invention employs a cassetteless vessel with an internal cavity defined by an internal surface for accommodating a plurality of substrate wafers and one or more standard cleaning solutions. The wafers are supported within the vessel by mounting members to minimize surface contact and have all surfaces of the wafers exposed to the cleaning liquid and the sonic waves. A number of megasonic wave generating transducers are placed around the exterior of the vessel. The transducers are staggered on opposite sides of the vessel such that the transducer beams are not transmitted through the mounting members, thereby allowing the transmitted beams to collectively envelop the entire surface area of the wafers without energy dissipation caused by intervening structure.